There is increasing demand for detection devices, especially in the automotive sector. For detecting the surroundings of a vehicle, the use of ultrasonic systems is preferred.
From German Patent Application No. 196 45 339, an ultrasonic parking assistance system and a method for a distance measurement as a function of vehicle data in a motor vehicle are known. This system emits a warning signal if an echo signal that lies inside a time window establishing a measuring range exceeds a specific threshold value. According to the German laid-open document, a static threshold value characteristic curve having a predefined number of interpolation points is transmitted via a transmitter/receiver line from a control unit to a sensor, typically at the beginning of the parking operation. A static threshold may be configured under two aspects: First, one that is optimized with respect to a minimal false alarm by noise and interference sources, which goes hand in hand with a high threshold level and entails low sensitivity or range of the detection system as a function of the reflecting size of the object to be detected. Secondly, there is the possibility of providing high sensitivity of the system by setting the threshold close to the noise/clutter limit, which results in a system exit due to “phantom” obstacles by a temporarily occurring interference source or by temporary interference influences.
Clutter or ground clutter refers to reflections of an ultrasound signal due to gravel or crushed stones, for instance. As a consequence, the configuration of a static threshold value characteristic curve involves a compromise between the aspects of high sensitivity, if possible, and a still acceptable false alarm rate under worst-case ambient conditions. Noise is caused by electronic amplification of the reception of the transducer signal and determines the best-case sensitivity limit or the lowest possible threshold.
The use of an adaptive threshold on the basis of a binary, i.e., digital, analysis on the level of the control unit is not possible either, for two reasons: First of all, the track evaluations in the control unit and a dead time arising in the transmission are incompatible with the latency-time demands of expanded detection-system functions such as autonomous driveaway or detection in the region of the blind spot. On the other hand, the transmitted time-analog, binary-valued information from the sensor, i.e., the transmitter/receiver unit, to the control unit, includes only a radial obstacle or clutter distance, but no quantization of the magnitude or the amount of the clutter or noise level. No suitable adaptive threshold adaptation in the control unit is possible on this basis, since no differentiation criterion between a target object and clutter, i.e., interference reflections, is provided.
However, the expanded functions of the detection system should also be utilizable in a dynamic environment in which the acoustic interference level may change rapidly, for instance due to passing interferences such as two-stroke motorcycles, wind noise and/or so-called microphonics, which depend on the engine load or engine speed or other vehicle vibrations.